The present invention relates to circuits for transmitters of radio communications, below denoted as “transmission circuits”.
In general, to process modulated signals with amplitude modulation, especially multiple modulation such as Quadrature Amplitude Modulation (QAM), radio-frequency power amplifiers in transmission circuits for transmitting power to antennas need linear amplification. Therefore, class A or AB had been adopted as operation classes of the RF power amplifiers.
However, with the progress of broadband communication, communication methods using subcarrier modulation such as Orthogonal Frequency Division Multiplex (OFDM) have come to be used and high efficiency is not expected any more with the conventional class-A or AB radio frequency amplifiers. That is, in the OFDM modulation, subcarriers are overlapped so that a large amount of power is instantaneously generated at random time and the ratio between the average power and the peak power, i.e., a Peak to Average Power Ratio (PAPR), is high. Therefore, a large amount of DC power always needs to be held in order to allow linear amplification of peak power which is much greater than the average power. With respect to class-A operation, the efficiency is only 50% at the maximum. In particular, in the case of the OFDM modulation, since the PAPR is high, DC power obtained by multiplying current by the difference between the peak voltage for the peak power and the peak voltage for the peak power is lost in the form of heat in almost all the time except for the period in which the peak power is output. Accordingly, the resultant efficiency decreases largely.
As a result, wireless equipments using batteries as their power supplies, for example, have shorter continuous operation time, and thus inconveniences arise in actual application.
To solve this problem, a conventional Envelope Elimination and Restoration (EER) technique known as Kaln Technique was proposed in FIG. 6 of U.S. Pat. No. 6,256,482B1, for example.
FIG. 8 is a circuit block diagram showing a general concept of a known EER technique. In FIG. 8, an OFDM modulated wave (modulated signal) generated by a modulator 401 is input to a detector 402, which is a modulated signal detecting means, through a modulated signal line 410. The detector 402 detects the OFDM modulated wave by dividing the wave into a phase component and an amplitude component. Specifically, the IQ carriers of the OFDM modulated wave generated by the modulator 401 are divided into an amplitude component √(I2+Q2) and a phase component tan−1 (Q/I) and these components are detected. The phase component (conjugated phase modulated wave) is transmitted from the detector 402 through phase component lines 411, is up-converted by a phase component quadrature modulator 404, and then is input to an RF input terminal of a PA 405 (RF power amplifier) in the form of RF signal power. The amplitude component (amplitude modulated wave) is transmitted through an amplitude component line 412, is subjected to DC-to-DC conversion in a DC-to-DC converter 403, and then is input to a power-supply-voltage terminal of the PA 405.
Now, as an example of the OFDM modulated wave in the case of IEEE 802.11a, about 7 dB is needed as a backoff (the amount representing how much lower the output power is than the saturation power). That is, only 20% of the RF peak power is used as the RF output power, so that the efficiency deteriorates from 50% to 10%. To use the class-A or AB PA with high efficiency, the minimum power-supply voltage required to output RF power is successively applied to the PA and the backoff is ideally 0 dB.
To solve this problem, in the EER technique, a modulated signal wave in which a quadrature modulation has been performed on a phase component obtained by polar mapping of a modulated signal is applied to the PA 405. The modulated wave with this phase component is a sine wave whose amplitude is constant and whose phase changes with time, so that the PA 405 is operable with a backoff of almost 0 dB. As the output of the PA 405, the amplitude component input from the power-supply-voltage terminal is multiplied by the phase component, so that the same modulated wave as that obtained by performing the quadrature modulation on the original modulated signal is obtained.